The present invention relates to a reflecting film excellent in cohesion resistance and sulfur resistance for lighting fixtures for the interior and exterior illumination of residential and office buildings or for headlamps for automobiles and trains.
Ag thin films or Ag-base alloy thin films have been used as reflecting films coating the inside surfaces of reflectors for lighting fixtures for the interior and exterior illumination of residential and office buildings or for headlamps for automobiles and trains. Ag thin films and Ag-base alloy thin films have high reflectance with visible light rays.
Ag is characterized by easy cohesion. Cohesion of Ag atoms reduces the reflectance of the reflecting film. The cohesion of Ag is caused by the adsorption of halogen ions and moisture contained in the atmosphere by the Ag and Ag-base alloy thin films. Therefore, Ag and Ag-base alloy thin films are coated with various resin thin films and coating films, respectively, to prevent the cohesion of Ag. However, the cohesion of Ag is caused often by halogen ions and moisture reaching the Ag and the Ag-base thin films through pinholes in the resin thin films and coating films. Cohesion of Ag forms innumerable white spots in the surfaces of Ag and Ag-base alloy thin films and causes color changes in Ag and Ag-base alloy thin films. Formation of white spots and color changes not only reduce reflectance but also spoils aesthetic appearance and deteriorate commercial value.
The reflecting film has a problem in addition to the problem of cohesion. The reflecting film is exposed to heat of high temperatures between 80 and 200° C. generated by a lamp when the lamp is lighted. Then, sulfur (S) contained in the atmosphere and diffused into the reflecting film reacts with the Ag of the reflecting film. Consequently, the surface of the reflecting film is sulfurated gradually and becomes dark and the reflectance of the reflecting film decreases.
Ag has been widely used as a material of electrical contacts and decorative films and the sulfur resistance of electrical contacts and decorative films has been improved by various methods including methods using alloys and those using multilayer films. For example, an Ag-base alloy disclosed in JP-A-55-85646 contains at least two elements among Pd, Au and Pt as additional element in a total content between 10 and 60 wt %. A multilayer film disclosed in JP-A-5-47251 has an Ag—Sn alloy base layer, and an Au or Au-base alloy thin film of a thickness between 10 and 200 nm deposited on the Ag—Sn alloy base layer. The Ag-base alloy and the multilayer film are excellent in sulfur resistance and are useful for forming electrical contacts and decorative films. However, the alloying elements contained in high alloying element contents and the Au thin film deposited on the surface of the Ag—Sn base layer reduces reflectance. Therefore, the Ag-base alloy is not suitable for forming the reflecting film and the multilayer film is unsuitable for use as a reflecting film.